1. Field of the Invention
This invention relates to the mixture with a solid combustible gas generant composition of an ingredient for the rapid generation of an exceedingly clean gas that is substantially free of combustion particle residue and extraneous noxious and offensive odors. The invention has particular utility in gas generators or inflators used for the generation of nitrogen gas to inflate vehicle inflatable cushion or air bag restraint systems to protect the occupants, passengers as well as the driver, from severe impact and possible injury during a collision.
2. Description of the Prior Art
In the prior art the amount of combustion particle residue or particulate leaving the inflator, when fired, has posed a problem. This particulate is due to liquids and gases that are produced by the pyrotechnics during combustion of a solid gas generant composition. Various chemical and mechanical cooling and filtering arrangements that have been proposed have been incapable of reducing the particle residue contained in the inflation gas to a desirably low level for the avoidance of discomfort to the vehicle occupants who are intended to be spared severe impact during a collision.
It is the practice to provide such filtering arrangements in a separate chamber in the gas flow path between the combustion chamber and the inflator outlet. Typical United States patents disclosing such filtering arrangements are U.S. Pat. No. 3,985,076, granted Oct. 12, 1976 and U.S. Pat. No. 4,296,084, granted Oct. 20, 1981, both of which patents have been assigned to the assignee of the present invention.
Gas generating compositions have been proposed that include mixtures of metal azides, oxidant metal compounds, and oxides of materials such as silicon dioxide. The oxide is said to react with and transform the toxic solid combustion residue to a non-toxic or physiologically harmless residue, specifically a glasslike alkali silicate when the oxide is silicon dioxide. United States patents that disclose such compositions are U.S. Pat. No. 3,883,373, granted May 13, 1975 and U.S. Pat. No. 3,947,300, granted Mar. 31, 1976. In U.S. Pat. No. 3,883,373 the components or ingredients of the gas generating composition are described as being employed in particulate form, in particle size less than 100 mesh Tyler screen size. The components in U.S. Pat. No. 3,947,300 are described as being ground and pulverized as finely as possible and highly dispersed and then compressed to form the composition. Such compositions leave something to be desired because of difficulty in uniformly mixing silicon dioxide with the metal azide and oxidant compound as required to react the silicon dioxide with the residues and for obtaining satisfactory combustibility.
Gas generant compositions that burn with the production only of the desired gaseous product and a solid product in the form of a sinter or clinker that is retained in the generator housing have also been proposed in the prior art. Such compositions are described in several United States patents as follows: U.S. Pat. No. 3,895,098, granted July 15, 1975, U.S. Pat. No. 3,931,040, granted Jan. 6, 1976, U.S. Pat. No. 3,996,079, granted Dec. 7, 1976, and U.S. Pat. No. 4,062,708, granted Dec. 13, 1977. In the disclosures of these patents the compositions are described as comprising mixtures of metal oxides such as nickel oxide or iron azide, and an alkali metal azide. A particle size for the reactant oxide in the range of a small fraction of a micron to a few microns is indicated as essential for effecting a burning rate fast enough for inflating an inflatable occupant restraint system.
Efforts to make an operative gas generator utilizing gas generating compositions as described above have been unsuccessful. The problems encountered include difficulty in compacting the mixture to form a stable pellet and difficulty in igniting the mixture.
It has been found that pelletizing the gas generating composition is essential for the composition to remain reliable over extended periods of ten (10) years or more, such as are involved in the useful life of a vehicle in which an air bag protective restraint system is intended to be installed, and for providing a uniform surface area for uniform burning upon ignition of the composition. Otherwise the burning rate is not predictable. Additionally, without pelletizing there is a tendency for packing and separation of the finely divided particles after the gas generator has been subjected to vibration over an extended period of time, as occurs during ordinary use, particularly when installed on a vehicle.
Gas generant compositions in pelletized form that have been found to be suitable for the rapid generation of nitrogen gas as required for the deployment of vehicle air bag restraint systems are disclosed in several United States patents that are assigned to the assignee of the present invention. These include U.S. Pat. No. 4,203,787, granted May 20, 1980 and U.S. Pat. No. 4,369,079, granted Jan. 18, 1983, the disclosures of which patents are incorporated herein by reference. While the gas generating pelletized material disclosed in these patents meet the requirements of burning rate, non-toxicity, and flame temperature, the amount of particle combustion residue in the generated gas is greater than is desirable, at least for some applications, even with the use of filtering arrangements provided in the gas flow path between the combustion chamber and the generator output, and has continued to pose a problem.
Thus, there still exists a need and a demand for further improvement in solid fuel gas generators or inflators particularly for the generation of clean nitrogen gas that is substantially free of combustible residue for inflating vehicle inflatable cushion or air bag restraint systems, or stated more specifically, to reduce the amount of particle combustible residue that flows out of the generator or inflator with the generated nitrogen gas into the inflatable cushion or air bag.